1. Field of the Invention
The invention relates to a gas sensor with an open optical measurement path.
2. Description of the Related Art
One such device is known for instance from U.S. Pat. No. 5,591,975, in which a device for measuring the exhaust gases of automobiles driving past it is described. The vehicles traverse the measurement path, which is bounded on one by a light source and on the other by a field of photodiodes.
In environmental analysis and industrial monitoring, the analysis of gaseous mixtures has gained increasing significance. There is accordingly increasing interest in the development of novel gas sensors that are optimized with respect to their sensitivity, selectivity, service life, and ease of manipulation.
Along with gas sensors that monitor a spatially narrowly limited region, recently gas sensors that monitor a larger area are increasingly being employed. The so-called gas sensors with an open optical measurement path (or open path sensors) record the mean concentration of the target gas over a path with a length of ten meters to a few hundred meters.
U.S. Pat. No. 5,339,155 describes a device in which the light of a light source is modulated in its wavelength, and this frequency modulation is converted, in the presence of the target gas, into an amplitude modulation that can be measured by a detector. The path is bounded, that is, defined, by a light source unit and a detector unit that are spatially separate from one another.
As light sources, laser light sources have increasingly been used in recent years. DFB laser diodes, in particular, are distinguished in that on the one hand the wavelength of the emitted light is very much narrower in its band than the absorption lines of gases, and on the other, this wavelength can be varied both by way of the temperature of the laser diode and via the triggering laser diode current.
In many laser diode-supported systems, so-called derivative spectroscopy is employed. In it, the wavelength of the laser diode is initially set, for instance by specifying the laser diode temperature, in such a way that the very narrow-band laser line is located spectrally within the absorption of a single gas line, for instance, of the target gas. The desired monitoring of the laser diode temperature can be performed for example by locating the laser diode chip on a Peltier element, which can be brought to a desired temperature by varying the Peltier current. The laser diode is operated with a current modulation in such a way that the gas line is periodically swept at the frequency f, and the modulation is preferably sinusoidal. Not only is the laser diode varied in its wavelength, but furthermore, as a parasitic effect, an amplitude modulation of the radiation intensity at the frequency f, the so-called 1f component, also occurs, and this amplitude modulation can be utilized to standardize the intensity.
Once the measurement path has been traversed, the intensity of the light is detected with a detector sensitive to the light of the light source; the detector generates an electrical signal that is proportional to the incident light intensity. This detector is equipped with an optical filter, which filters out interfering components of the spectrum, such as incident daylight. In the absence of the target gas, the detector signal is likewise sinusoidal at the frequency f, because of the corresponding amplitude modulation of the laser diode current. If target gas is present within the measurement path, however, then the intensity measured by the detector after the path has been traversed includes, as a function of time, components that are modulated with n-times the frequency, which are known as nth harmonic components or nth harmonics. The generation of these harmonic components is dictated by the nonlinear curvature of the absorption line of the gas. With the aid of suitable phase-sensitive measuring amplifiers (known as lock-in amplifiers], these harmonic components of the detector signal can be determined. While the 1f component of the detector signal is influenced hardly at all by the gas concentration, the higher 2f, 3f and further components are approximately proportional to the gas concentration. Thus the quotient of the 2f component and the 1f component, for instance, known as the 2f:1f quotient, represents a standardized number for determining the gas concentration that is independent of such external effects as aging of the light sources, and broad-band attenuation from dirt, fog, and so forth.
To compensate for zero drift and to increase the sensitivities, the fast 1f modulation of the laser diode wavelength is additionally underlayed by a slower modulation of the mean wavelength at the frequency F (f greater than F), by varying the laser diode current accordingly. This slow modulation can for instance be in the form of a linear tuning ramp (sawtooth); one period of this slow modulation is called xe2x80x9cscanxe2x80x9d. During one scan, a previously defined number of n 2f:1f quotients at n different wavelengths is picked up. The amplitudes of both the fast f- and the slow F-modulation of the wavelength are each selected such that they correspond approximately to the width of the gas line. Thus instead of the single value of a 2f:1f quotient described previously for a fixed wavelength, a plurality or a tuplet of n 2f:1f quotients at n different wavelengths is now obtained. This measurement value tuplet can serve, with suitable mathematical evaluation, for instance by a PCA (Principal Components Analysis) process or the like, both to determine the target gas concentration and to identify the target gas with certainty.
To prevent the originally set temperature of the laser diode or the Peltier element from varying during operation and thus varying the wavelength of the laser light, a beam splitter is mounted on the side of the light source unit or the detector unit; it deflects part of the light emerging from the laser diode through a cell (reference cell) in which a gas of suitable absorption capacityxe2x80x94for instance, the target gas itselfxe2x80x94is confined. This portion of the light is detected, after passing through the reference cell, by a light-sensitive detector. With the aid of a phase-sensitive measuring amplifier, analogously to the measurement tuplet, a set of reference measurement values, a so-called reference tuplet, can be determined that is preferably again composed of 2f:1f quotients. By a comparison of this reference tuplet with values stored in memory, it is possible to detect any wavelength drift and to correct the temperature of the laser diode in such a way that this wavelength drift is precisely compensated for.
Open path gas sensors, when there is a large three-dimensional spacing between the light source unit and the detector unit, can be manipulated upon assembly and in operation only with difficulty and at great effort. For instance in derivative spectroscopy, problems of adaptation arise between the control of the light source and the evaluation of the detector signals. As much as possible, adaptations between the control of the light source and the evaluation for the detector signals must be done in a separate control and evaluation device, which is in communication with both the light source unit and the detector unit.
Such adaptation and control problems can be partially avoided by gas sensor systems in which the light source unit and detector unit are disposed directly adjacent one another and a long open measurement path is realized by providing that the light source aims a measurement beam at a remotely located retroreflector, by which the measurement beam is reflected and thrown back at the detector unit. One such gas sensor is known from German Patent DE 196 11 290 C2, for instance. In this known gas sensor device, the light source unit and the detector unit are not accommodated separately from one another but rather in one housing. This gas sensor has the advantage that the light source unit and the detector unit can function, adapted to one another, in a simple way. A disadvantage of such gas sensors is the use of a retroreflector, because then the signal-to-noise ratio is less favorable than in gas sensors in which the measuring light from the light source is aimed directly at the detector unit.
Another problem in gas sensors with open paths of great length resides in calibrating the gas sensor. Since the light source unit and the detector unit can be several hundred meters apart, both in assembly and when the system is put into operation careful optical alignment of the two units with one another must first be done. This raises the following problems. First, the optical guide elements, which in both the light source unit and the detector unit serve to project the measurement light at the detector, are mounted rigidly in the light source unit and detector unit. Thus the entire light source unit and the entire receiver device must be moved to enable optimal calibration of the system. If the spacing between the light source unit and the detector unit is 100 meters, for instance, then the light source unit must be capable of being moved with a precision of 1/20xc2x0, if the measurement light in the detector unit is to be positionable with an accuracy of 10 cm. This demand for precision, along with the typical weight of the light source unit, which is several kilograms, is a major engineering problem in practical terms. Secondly, during the calibration of the system, information as to how good the calculation is at the moment is not immediately available. Thus either a course adjustment must be made first, using an optical aid such as a telescopic sight and then monitoring the detector signal at the detector 100 meters away, in which case either the mechanic himself must go to the remote detector unit, or else a second person has to record and monitor the detector signal there. Thus puffing a gas sensor with an open path of great length into operation or monitoring its calibration is possible only at high expense in terms of both labor and time.
It is therefore the object of the present invention to create a more easily manipulated and operated gas sensor with an optical measurement path that is easily adjustable even if the path is long. Furthermore, the measurement light of the light source unit should be aimed directly at the detector unit, rather via a retroreflector, so that a good signal-to-noise ratio is attainable, and at the same time simple adaptation between the detector unit and the light source unit should be made possible.
According to the invention, the detector unit is provided with a transmitter device and the light source unit is provided with a receiver device that responds to the signals of the transmitter device, so that a direct data exchange between the detector unit and the light source unit is made possible; the transmitter and receiver devices each communicate with control and evaluation devices in the detector unit and the light source unit. In this way it is possible, for instance, upon calibration of the gas sensor to undertake an optimization of the orientation of the light source unit and the detector unit to one another; the light source unit receives feedback of the detector signal detected from the detector unit, so that a scanning surge makes it possible to optimize the intensity of the detector signal, thus enabling optimal aiming of the measuring light beam at the detector unit. Such a feedback can also be used to perform an automatic calibration of the light source unit and detector unit. To that end, electrically triggerable, movable optical guide elements (mirrors) can be present in both the light source unit and in the detector unit and are adjusted by control and evaluation devices in such a way that the highest possible detector signal results, and thus the best possible orientation or aiming of the detector in the beam path of the measuring light beam is attained.
In an advantageous embodiment, conversely, the light source unit is also equipped with a transmitter device and the detector unit is also equipped with a receiver device that responds to this transmitter device, so that a direct bidirectional data exchange between the light source unit and the detector unit is made possible. The capability of a bidirectional data exchange between the light source unit and the detector unit has many advantages. For instance, servicing functions, calibration of the gas sensor, or a self-test can be performed by means of suitably designed control and evaluation devices in the light source unit and the detector unit. Especially in conjunction with the above-described derivative spectroscopy in combination with a laser diode, it is highly advantageous from the standpoint of measurement technology if a bidirectional data exchange between the light source unit and the detector unit is achieved, for instance for the sake of demodulating the detector signal or for synchronizing the control and evaluation device in the detector unit with the tuning ramp that triggers the laser diode and that is monitored by the control and evaluation device in the light source unit.
The transmitter and receiver devices in the light source unit and the detector unit can function either in wireless fashion or via a cable connection, either an electrical or an optical cable connection. Alternatively, it is also possible for the transmitter device in the light source unit to be realized by,. means of a suitable design of the control and evaluation device, which triggers the light source in such a way that the measured light is modulated in frequency and/or amplitude, so that by means of the measuring light, data can also be transmitted that can be demodulated from the detector signal by a suitably designed control and evaluation device in the detector unit.
The light source may be an incandescent lamp, flash bulb, laser diode or any other light source. The measuring light is not limited to the range of visible light, for instance; in many cases it is advantageous to work outside the range of visible light, for instance in the infrared range, so that interfering effects from background light and sunlight can be precluded.